1. Technical Field
The present invention relates to an atomic oscillator, an electronic apparatus, and a moving object.
2. Related Art
As shown in FIG. 13, a cesium atom, which is a kind of alkali metal atom, has a ground level of 6S1/2 and two excitation levels of 6P1/2 and 6P3/2, and furthermore, the respective levels of 6S1/2, 6P1/2, and 6P3/2 have ultrafine structures with a plurality of split energy levels. Specifically, 6S1/2 has two ground levels F=3 and 4, 6P1/2 has two excitation levels F′=3 and 4, and 6P3/2 has four excitation levels F′=2, 3, 4, and 5.
For example, a cesium atom in a ground level F of 6S1/2=3 can shift to one of the excitation levels F′ of 6P3/2=2, 3, and 4, by absorbing a D2 beam while the cesium atom cannot shift to the excitation level F′=5. A cesium atom in the ground level F of 6S1/2=4 can shift to one of the excitation levels F′ of 6P3/2=3, 4, and 5 by absorbing the D2 beam while the cesium atom cannot shift to the excitation level F′=2. This is based on a transition selection rule on an assumption of electric dipole transition. In contrast, a cesium atom in one of excitation levels F′ of 6P3/2=3 and 4 emits the D2 beam and can shift to a ground level F of 6S1/2=3 or 4 (either an original ground level or the other ground level). Here, the three levels (the two ground levels and one excitation level) consisting of the two ground levels F of 6S1/2=3 and 4 and one of the excitation levels F′ of 6P3/2=3 and are referred to as Λ-type three levels since Λ-type transition by absorption and light emission of the D2 beam is possible. Similarly, the three levels consisting of two ground levels F of 6S1/2=3 and 4 and one of the excitation levels F′ of 6P1/2=3 and 4 can realize the Λ-type transition by absorption and light emission of a D1 beam and therefore forms the Λ-type three levels.
On the other hand, a cesium atom in an excitation level F′ of 6P3/2=2 emits the D2 beam and always shifts to a ground level of 6S1/2=3 (original ground level), and similarly, a cesium atom in an excitation level F′ of 6P3/2=5 emits the D2 beam and always shifts to a ground level F of 6S1/2=4 (original ground level). That is, the three levels consisting of the two ground levels F of 6S1/2=3 and 4 and the excitation level F′ of 6P3/2=2 or 5 cannot realize the Λ-type transition by absorption and emission of the D2 beam and therefore do not form the Λ-type three levels. In addition, it has been known that alkali metal atoms other than the cesium atom similarly have two ground levels and an excitation level which form the Λ-type three levels.
Incidentally, if an alkali metal atom in the form of a gas is irradiated simultaneously with resonance light (referred to as resonance light 1) with a frequency (the number of vibrations) corresponding to an energy difference between the first ground level (the ground level F of 6S1/2=3 in the case of a cesium atom) which forms the Λ-type three levels and the excitation level (the excitation level F′ of 6P3/2=4, for example, in the case of the cesium atom) and with resonance light (referred to as resonance light 2) with a frequency (the number of vibrations) corresponding to an energy difference between the second ground level (the ground level F of 6S1/2=4 in the case of the cesium atom) and the excitation level, it is known that a state where the two ground levels are overlapped, namely a quantum coherence state (dark state) is created and that an electromagnetically induced transparency (EIT) phenomenon (also referred to as a coherent population trapping (CPT)) in which excitation to the excitation level is stopped occurs. A frequency difference between a resonance light pair (the resonance light 1 and the resonance light 2) which causes the EIT phenomenon completely coincides with a frequency corresponding to an energy difference ΔE12 between the two ground levels of the alkali metal atom. In the case of the cesium atom, for example, a frequency corresponding to the energy difference between the two ground levels is 9.192631770 GHz, and therefore, the EIT phenomenon occurs if the cesium atom is irradiated simultaneously with two types of laser light, namely the D1 light and the D2 light with a frequency difference of 9.192631770 GHz.
Accordingly, intensity of light which penetrates through the alkali metal atom steeply changes depending on whether or not light with a frequency ω1 and light with a frequency ω2 function as the resonance light pair and the alkali metal atom causes the EIT phenomenon when the alkali metal atom in the form of a gas is irradiated simultaneously with the two light waves as shown in FIG. 14. A signal indicating intensity of transmitted light, which steeply changes, is referred to as an EIT signal (resonance signal), and a level of the EIT signal indicates a peak value when the frequency difference ω1-ω2 between the resonance light pair completely coincides with the frequency ω12 corresponding to ΔE12. Thus, it is possible to realize an oscillator with high accuracy by controlling a light detector so as to detect a peak top of the EIT signal, such that the frequency difference ω1-ω2 of the two light waves completely coincides with the frequency ω12 corresponding to ΔE12 by irradiating an atom cell (gas cell) in which the alkali metal atom in the form of gas is encapsulated with the two light waves. Such a technology relating to the atomic oscillator is disclosed in U.S. Pat. No. 6,320,472, for example.
According to an atomic oscillator of the EIT scheme, a semiconductor laser generates light, a center frequency f0 of which is modulated with a modulation frequency fm, by superimposing a modulation signal with the frequency fm on the bias current determining the center frequency f0 (=v/λ0: v represents a velocity of light, λ0 represents a center wavelength) (carrier frequency) of the light generated by the semiconductor laser and supplying the superimposed modulation signal to the semiconductor laser, for example. The gas cell is irradiated with the light emitted by the semiconductor laser, and light which penetrates through the gas cell is detected by a light detector. An oscillation frequency of a voltage controlled crystal oscillator (VCXO) is controlled in accordance with intensity of the light detected by the light detector, and a modulation signal with a frequency fm is generated via a Phase Locked Loop (PLL) circuit. Then, control is performed such that first-order sideband light which is emitted by the semiconductor laser, namely light with a frequency f0+fm and light with a frequency f0−fm forms a resonance light pair. By such control, a frequency deviation of an output signal of the voltage controlled crystal oscillator (VCXO) is significantly reduced, and it is possible to realize an oscillator with high frequency accuracy.
Since frequency accuracy (short-term stability) is further enhanced as S/N of an EIT signal increases, it is desirable that the center wavelength λ0 of the light emitted by the semiconductor laser be adjusted to an optimal wavelength so as to maximize an amount of light absorption of the gas cell. Thus, JP-A-2011-101211 proposes an atomic oscillator capable of pushing a direct current bias current to a point at which the intensity of the transmitted light that penetrates through the cell encapsulating the alkali metal atom is minimized, for example.
However, if a long time (ten years, for example) has passed, there is a case where a bias current value when the amount of light absorption of the gas cell reaches the maximum value (the minimum point of an absorption band) significantly deviates from an initial setting value due to time degradation of the semiconductor laser. In addition, since the absorption band has two absorption bottoms in practice, there is a possibility in that the direct current bias current is pushed to a bottom with a smaller absorption amount depending on a size and a direction of the deviation of the bias current value from the initial setting value in the case of the atomic oscillator in the related art as disclosed in JP-A-2011-101211.